Elemental characterisation of the pyramidal neuron layer within the rat and mouse hippocampus.

A unique combination of sensitivity, resolution, and penetration make X-ray fluorescence imaging (XFI) ideally suited to investigate trace elemental distributions in the biological context. XFI has gained widespread use as an analytical technique in the biological sciences, and in particular enables exciting new avenues of research in the field of neuroscience. In this study, elemental mapping by XFI was applied to characterise the elemental content within neuronal cell layers of hippocampal sub-regions of mice and rats. Although classical histochemical methods for metal detection exist, such approaches are typically limited to qualitative analysis. Specifically, histochemical methods are not uniformly sensitive to all chemical forms of a metal, often displaying variable sensitivity to specific "pools" or chemical forms of a metal. In addition, histochemical methods require fixation and extensive chemical treatment of samples, creating the strong likelihood for metal redistribution, leaching, or contamination. Direct quantitative elemental mapping of total elemental pools, in situ within ex vivo tissue sections, without the need for chemical fixation or addition of staining reagents is not possible with traditional histochemical methods; however, such a capability, which is provided by XFI, can offer an enormous analytical advantage. The results we report herein demonstrate the analytical advantage of XFI elemental mapping for direct, label-free metal quantification, in situ within ex vivo brain tissue sections. Specifically, we definitively characterise for the first time, the abundance of Fe within the pyramidal cell layers of the hippocampus. Localisation of Fe to this cell layer is not reproducibly achieved with classical Perls histochemical Fe stains. The ability of XFI to directly quantify neuronal elemental (P, S, Cl, K, Ca, Fe, Cu, Zn) distributions, revealed unique profiles of Fe and Zn within anatomical sub-regions of the hippocampus i.e., cornu ammonis 1, 2 or 3 (CA1, CA2 or CA3) sub-regions. Interestingly, our study reveals a unique Fe gradient across neuron populations within the non-degenerating and pathology free rat hippocampus, which curiously mirrors the pattern of region-specific vulnerability of the hippocampus that has previously been established to occur in various neurodegenerative diseases.

Visualising coordination chemistry: fluorescence X-ray absorption near edge structure tomography.

Coordination chemistry underlies the structure/function of biological metal complexes. Contextualising this chemical information within an organism's physiology is critical for enhancing the understanding of bioinorganic chemistry but few high-fidelity probes are available. Here we develop fluorescence X-ray absorption near-edge structure tomography as a means for studying the spatial arrangement of biological coordination chemistry within intact organisms, and demonstrate the approach by mapping the distribution of cuprous and cupric complexes within Drosophila melanogaster.

Calculation of Projected Bond-Orientational Order Parameters to Quantify Local Symmetries from Transmission Diffraction Data.

The bond-orientational order parameters introduced by Steinhardt et al. [Phys. Rev. B 28, 784 (1983)] have been an invaluable measurement tool for assessing short-range order in disordered, close-packed assemblies of particles in which the particle positions are known. In many glassy systems the measurement of particle position is not possible or limited (field of view, thickness, resolution) and the bond-orientational order parameters cannot be measured, or adequately sampled. Here we calculate a set of rotationally averaged, projected bond-orientational order parameters that reflect the symmetries of close-packed particle clusters when projected onto a plane. We show by simulation that these parameters are unique fingerprints that can be directly compared to angular correlations in limited-volume, transmission geometry, diffraction patterns from close-packed glassy assemblies.

Publisher's Note: Calculation of Projected Bond-Orientational Order Parameters to Quantify Local Symmetries from Transmission Diffraction Data [Phys. Rev. Lett. 116, 205501 (2016)].

This corrects the article DOI: 10.1103/PhysRevLett.116.205501.

Molar concentration from sequential 2-D water-window X-ray ptychography and X-ray fluorescence in hydrated cells.

Recent developments in biological X-ray microscopy have allowed structural information and elemental distribution to be simultaneously obtained by combining X-ray ptychography and X-ray fluorescence microscopy. Experimentally, these methods can be performed simultaneously; however, the optimal conditions for each measurement may not be compatible. Here, we combine two distinct measurements of ultrastructure and elemental distribution, with each measurement performed under optimised conditions. By combining optimised ptychography and fluorescence information we are able to determine molar concentrations from two-dimensional images, allowing an investigation into the interactions between the environment sensing filopodia in fibroblasts and extracellular calcium. Furthermore, the biological ptychography results we present illustrate a point of maturity where the technique can be applied to solve significant problems in structural biology.

Characterization of an indirect X-ray imaging detector by simulation and experiment.

We describe a comprehensive model of a commercial indirect X-ray imaging detector that accurately predicts the detector point spread function and its dependence on X-ray energy. The model was validated by measurements using monochromatic synchrotron radiation and extended to polychromatic X-ray sources. Our approach can be used to predict the performance of an imaging detector and can be used to optimize imaging experiments with broad-band X-ray sources.

Systematic mapping of icosahedral short-range order in a melt-spun Zr36Cu64 metallic glass.

By analyzing the angular correlations in scanning electron nanodiffraction patterns from a melt-spun Zr(36)Cu(64) glass, the dominant local order was identified as icosahedral clusters. Mapping the extent of this icosahedral short-range order demonstrates that the medium-range order in this material is consistent with a face-sharing or interpenetrating configuration. These conclusions support results from atomistic modeling and a structural basis for the glass formability of this system.

A multi-technique investigation of copper and zinc distribution, speciation and potential bioavailability in biosolids.

The use of biosolids in agriculture continues to be debated, largely in relation to their metal contents. Our knowledge regarding the speciation and bioavailability of biosolids metals is still far from complete. In this study, a multi-technique approach was used to investigate copper and zinc speciation and partitioning in one contemporary and two historical biosolids used extensively in previous research and field trials. Using wet chemistry and synchrotron spectroscopy techniques it was shown that copper/zinc speciation in the biosolids was largely equivalent despite the biosolids being derived from different countries over a 50 year period. Furthermore, copper speciation was consistently dominated by sorption to organic matter whereas Zn partitioned mainly to iron oxides. These data suggest that the results of historical field trials are still relevant for modern biosolids and that further risk assessment studies should concentrate particularly on Cu as this metal is associated with the mineralisable biosolids fraction.

Trends in hard X-ray fluorescence mapping: environmental applications in the age of fast detectors.

Environmental samples are extremely diverse but share a tendency for heterogeneity and complexity. This heterogeneity poses methodological challenges when investigating biogeochemical processes. In recent years, the development of analytical tools capable of probing element distribution and speciation at the microscale have allowed this challenge to be addressed. Of these available tools, laterally resolved synchrotron techniques such as X-ray fluorescence mapping are key methods for the in situ investigation of micronutrients and inorganic contaminants in environmental samples. This article demonstrates how recent advances in X-ray fluorescence detector technology are bringing new possibilities to environmental research. Fast detectors are helping to circumvent major issues such as X-ray beam damage of hydrated samples, as dwell times during scanning are reduced. They are also helping to reduce temporal beamtime requirements, making particularly time-consuming techniques such as micro X-ray fluorescence (µXRF) tomography increasingly feasible. This article focuses on µXRF mapping of nutrients and metalloids in environmental samples, and suggests that the current divide between mapping and speciation techniques will be increasingly blurred by the development of combined approaches.

Differential phase contrast with a segmented detector in a scanning X-ray microprobe.

Scanning X-ray microprobes are unique tools for the nanoscale investigation of specimens from the life, environmental, materials and other fields of sciences. Typically they utilize absorption and fluorescence as contrast mechanisms. Phase contrast is a complementary technique that can provide strong contrast with reduced radiation dose for weakly absorbing structures in the multi-keV range. In this paper the development of a segmented charge-integrating silicon detector which provides simultaneous absorption and differential phase contrast is reported. The detector can be used together with a fluorescence detector for the simultaneous acquisition of transmission and fluorescence data. It can be used over a wide range of photon energies, photon rates and exposure times at third-generation synchrotron radiation sources, and is currently operating at two beamlines at the Advanced Photon Source. Images obtained at around 2 keV and 10 keV demonstrate the superiority of phase contrast over absorption for specimens composed of light elements.

Quantitative phase measurement in coherent diffraction imaging.

We demonstrate high spatial resolution phase retrieval of a non-periodic gold nano-structure using the method of Fresnel coherent diffractive imaging. The result is quantitative to better than 10% and does not rely on any a priori knowledge of the sample.

Quantitative phase imaging with a scanning transmission x-ray microscope.

We obtain quantitative phase reconstructions from differential phase contrast images obtained with a scanning transmission x-ray microscope and 2.5 keV x rays. The theoretical basis of the technique is presented along with measurements and their interpretation.

Experimental measurement of the four-dimensional coherence function for an undulator x-ray source.

A full measurement of the four-dimensional coherence function from an undulator beam line is reported. The analysis is based on the observation that the data are consistent with a coherence function that is mathematically separable. The effective source size can be altered by changing the width of the exit slit, and the complete coherence function is presented for two settings. We find, to within experimental error, that the four-dimensional complex degree of coherence can be described as a real Gaussian function that depends only on the difference of the spatial coordinates.

Fresnel coherent diffractive imaging.

We present an x-ray coherent diffractive imaging experiment utilizing a nonplanar incident wave and demonstrate success by reconstructing a nonperiodic gold sample at 24 nm resolution. Favorable effects of the curved beam illumination are identified.